1. Field of the Invention
This invention relates to a catalyst for purifying exhaust gases discharged from an internal combustion engine and a process for purifying the exhaust gases using the catalyst. More specifically, this invention relates to a catalyst for efficiently purifying hydrocarbons emitted at the start of the engine and a process for purifying the exhaust gases using the catalyst.
2. Description of Related Art
Technology of after-treatment by catalysts forms core of means to reduce exhaust gases (or pollutants) from an internal combustion engine. Expectations for improving the efficiency have increasingly grown from the view of environmental conservation. The exhaust gas of gasoline engines contains hydrocarbons (hereinafter “hydrocarbon” may be referred to as “HC”), carbon monoxide (CO), and nitrogen oxide (NOx). For purifying these pollutants, so-called “three-way catalyst or TWC” which has a noble metal and refractory inorganic oxides, such as alumina and cerium oxide, is in common usage.
The TWC is activated by the heat of exhaust gases, thereby promoting the purification reaction. When the temperatures of the exhaust gas or of the catalyst body are low, purification reactions do not proceed efficiently and the pollutant gas is discharged unpurified. Particularly, the unburned HC which comes out at the ignition of the engine in a cold state, is not readily purified (in spite of a high concentration) by a conventional TWC because the temperatures of the gas and body are low. Thus, so-called “HC-adsorber catalyst” system has been proposed which the unburned HC are adsorbed in a cold state by adsorbents, and then the HC are purified by a catalyst in a temperature in which the catalyst has been activated. It has been disclosed that a HC adsorber catalyst having structure of both a oxidation-reduction ability and zeolite are layered (for example, JP-A-02-056247) and that a purification system in which a HC adsorbent brick and a conventional catalyst brick are separately allocated (for example, U.S. Pat. No. 4,985,210A).